Variable focal length lens, photographing lens unit, camera, and portable information terminal device

ABSTRACT

A negative first group optical system, a positive second group optical system, and a positive third group optical system are sequentially arranged from the object side. A stop moving integrally with the second group optical system is provided on the object side of the second group optical system. The focal length is changed by changing distances between the group optical systems, and the third group optical system is moved on an optical axis. The first group optical system includes a negative meniscus lens, a negative meniscus lens, and a positive lens. The second group optical system includes a cemented lens of a positive lens and a negative lens, a positive lens, and a positive lens. The third group optical system includes one positive lens not including an aspherical surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present document incorporates by reference the entire contents ofJapanese priority document, 2003-191519 filed in Japan on Jul. 3, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of a variable focallength lens such as a zoom lens that is used as a photographing opticalsystem in various cameras including a so-called silver-salt camera. Inparticular, the present invention relates to a variable focal lengthlens that can be preferably used in cameras such as digital cameras andvideo cameras, and to a photographing lens unit, a camera, and aportable information terminal device that includes such a variable focallength lens.

2. Description of the Related Art

Recently, cameras such as digital cameras and electronic cameras havebecome common. Such a camera acquires a photograph of a subject imagewith a solid-state image pickup element such as a charge-coupled device(CCD) image pickup element to obtain image data of a still image or amoving image (movie image) and digitally records the image data in anonvolatile semiconductor memory or the like. A flash memory is anexample of the nonvolatile semiconductor memory. A traditional camera inwhich a conventional silver-salt film is used, that is, a silver-saltcamera is gradually becoming outdated.

A market for such a digital camera has grown to be extremely large, anddemands of users for the digital camera have been diversified. Aboveall, the users often demand for an improvement in image quality andminiaturization of the digital cameras.

To achieve the characteristics such as small size, light weight, andhigh performance, variable focal length lenses such as zoom lenses areoften used in the digital cameras. Such a zoom lens generally has atwo-lens group or three-lens group structure, i.e., a structure thatincludes only a few lenses. If the zoom lens includes lens groups havingseveral lenses, when the lenses are moved in focusing, the advantage ofminiaturization cannot be fully achieved, moreover, the operabilitybecome poor, because movement of a center of gravity of the lenses islarge. Therefore, sometimes the focusing is performed by moving onlysome of the lens groups.

For example, zoom lenses have been disclosed in Japanese PatentApplication Laid-Open Publication Nos. 2003-131134, 2003-107352, and2003-35868 as zoom lenses that can be preferably used in digital camerasand are suitable for miniaturization. A typical zoom lens includes afirst group optical system having a negative refracting power, a secondgroup optical system having a positive refracting power, and a thirdgroup optical system having a positive refracting power. The first tothe third group optical systems are sequentially arranged from an objectside. A stop is provided on the object side of the second group opticalsystem that moves integrally with the second group optical system. Thefocal length of the zoom lens can be changed by changing the distancebetween the respective group optical systems.

The first group optical system includes a negative meniscus lens, anegative meniscus lens, and a positive lens that are sequentiallyarranged from the object side. The second group optical system includesa positive lens, a negative lens, a positive lens, and a positive lensthat are sequentially arranged from the object side. The third groupoptical system includes one positive lens.

In the zoom lens disclosed in Japanese Patent Application Laid-OpenPublication No. 2003-131134, an image side surface of the negativemeniscus lens second from the object side of the first group opticalsystem, a surface on the most object side of the second group opticalsystem, a surface on the most image side of the second group opticalsystem, and a surface on the object side of the third group opticalsystem are formed as aspherical surfaces, respectively.

In another exemplary structure, the positive lens on the most objectside and the negative lens adjacent to the positive lens of the secondgroup optical system are formed as a cemented lens, and an image sidesurface of the negative meniscus lens second from the object side of thefirst group optical system, a surface on the most object side of thesecond group optical system, a surface on the most image side of thesecond group optical system, and a surface on the object side of thethird group optical system are formed as aspherical surfaces,respectively.

In still another exemplary structure, the positive lens on the mostobject side and the negative lens adjacent to the positive lens of thesecond group optical system are formed as a cemented lens, and an imageside surface of the negative meniscus lens second from the object sideof the first group optical system, a surface on the most object side ofthe second group optical system, and a surface on the object side of thethird group optical system are formed as aspherical surfaces,respectively.

In still another exemplary structure, the negative lens and the negativelens second from the mage side adjacent to the negative lens of thesecond group optical system are formed as a cemented lens, and an imageside surface of the negative meniscus lens second from the object sideof the first group optical system, a surface on the most object side ofthe second group optical system, a surface on the most image side of thesecond group optical system, and a surface on the object side of thethird group optical system are formed as aspherical surfaces,respectively.

In this way, in the technology disclosed in Japanese Patent ApplicationLaid-Open Publication No. 2003-131134, the image surface is corrected byusing the aspherical surface for the positive lens of the third groupoptical system. Japanese Patent Application Laid-Open Publication Nos.2003-107352 and 2003-35868 disclose similar structures.

Thus, in the conventional technology, the image surface is corrected byusing the aspherical surface for the positive lens of the third groupoptical system.

Although it is effective to use the aspherical surface for the thirdgroup optical system for correction of the image surface, deteriorationof image performance due to the focusing occurs when the third groupoptical system is moved along an optical axis for focusing.

This point is explained in more detail below. When the third groupoptical system is used for focusing, it is necessary to secure an amountof movement of the third group optical system. For securing the amountof movement of the third group optical system, one approach is toincrease the distance between the second and the third group opticalsystems or to increase a refracting power of the third group opticalsystem to reduce the amount of movement of the third group opticalsystem. However, the total length of the zoom lens increases and itbecomes bulky if the distance between the second and the third groupoptical systems is increased. On the other hand, the aberrationcorrection becomes difficult if the refracting power of the third groupoptical system is increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve at least the problemsin the conventional technology.

A variable focal length lens according to one aspect of the presentinvention includes a first group optical system having a negativerefracting power, a second group optical system having a positiverefracting power, and a third group optical system having a positiverefracting power, wherein the first through the third group opticalsystems are sequentially arranged from an object side; and a stopprovided on the object side of the second group optical system and thatmoves integrally with the second group optical system. A focal length ischanged by changing distances between the first through the third groupoptical systems and when performing focusing the third group opticalsystem is moved along an optical axis, the first group optical systemincludes a negative meniscus lens, a negative meniscus lens, and apositive lens those are sequentially arranged from the object side, atleast one surface of the two negative meniscus lenses being anaspherical surface, the second group optical system includes a cementedlens of a positive lens and a negative lens, a positive lens, and apositive lens those are sequentially arranged from the object side, asurface on the object side of the positive lens on the most object sidebeing an aspherical surface, and the third group optical system includesone positive lens not including an aspherical surface.

A variable focal length lens according to another aspect of the presentinvention includes a first group optical system having a negativerefracting power, a second group optical system having a positiverefracting power, and a third group optical system having a positiverefracting power, wherein the first through the third group opticalsystems are sequentially arranged from an object side; and a stopprovided on the object side of the second group optical system and thatmoves integrally with the second group optical system. A focal length ischanged by changing relative distances between the first through thethird group optical systems and when performing focusing the third groupoptical system is moved along an optical axis, the first group opticalsystem includes a negative meniscus lens, a negative meniscus lens, anda positive lens those are sequentially arranged from the object side,the second group optical system includes a cemented lens of a positivelens and a negative lens, a positive lens, and a positive lens those aresequentially arranged from the object side, the third group opticalsystem includes one positive lens, at least one surface of the negativemeniscus lens in the first group optical system and a surface on themost object side in the second group optical system being asphericalsurfaces, and the third group optical system includes only a sphericallens.

A photographing lens unit, a camera, a portable information terminaldevice according to still another aspect of the present inventioninclude the above variable focal length lens according to the presentinvention.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an optical system of a variable focal lengthlens according to a first embodiment of the present invention when thefocal length is wide-angle end;

FIG. 2 is a side view of the optical system in FIG. 1 when the focallength is intermediate focal length;

FIG. 3 is a side view of the optical system in FIG. 1 when the focallength is telescopic end;

FIG. 4 is an aberration curve diagram showing various aberrations at thewide-angle end of the variable focal length lens in FIGS. 1 to 3;

FIG. 5 is an aberration curve diagram showing various aberrations at theintermediate focal length of the variable focal length lens in FIGS. 1to 3;

FIG. 6 is an aberration curve diagram showing various aberrations at thetelescopic end of the variable focal length lens in FIGS. 1 to 3;

FIG. 7 is a side view of an optical system of a variable focal lengthlens according to a second embodiment of the present invention when thefocal length is wide-angle end;

FIG. 8 is a side view of the optical system in FIG. 7 when the focallength is intermediate focal length;

FIG. 9 is a side view of the optical system in FIG. 7 when the focallength is telescopic end;

FIG. 10 is an aberration curve diagram showing various aberrations atthe wide-angle end of the variable focal length lens in FIGS. 7 to 9;

FIG. 11 is an aberration curve diagram showing various aberrations atthe intermediate focal length of the variable focal length lens in FIGS.7 to 9;

FIG. 12 is an aberration curve diagram showing various aberrations atthe telescopic end of the variable focal length lens in FIGS. 7 to 9;and

FIG. 13 is a perspective view from a photographer side schematicallyshowing an external structure of a camera according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of a variable focal length lens, a photographinglens unit, a camera, and a portable information terminal device of thepresent invention will be hereinafter explained in detail with referenceto the accompanying drawings. The principle of the present inventionwill be explained first.

The variable focal length lens according to the present invention is, ingeneral, a zoom lens. This variable focal length lens includes a firstgroup optical system having a negative refracting power, a second groupoptical system having a positive refracting power, and a third groupoptical system having a positive refracting power that are sequentiallyarranged from an object side, and a stop provided on the object side ofthe second group optical system that moves integrally

TABLE 2 Combination Type of Information Number (1) (2) . . . (k) 1 y₁₁y₁₂ . . . y_(1k) 2 y₂₁ y₂₂ . . . y_(2k) . . . . . . n y_(n1) y_(n2) . .. y_(nk) Average 0 0 . . . 0 Standard Deviation 1 1 . . . 1

All correlation coefficients r_(pq)(=r_(qp)) between two combinations ofdata of k combinations of data are calculated using an expression (2),and are expressed by a matrix R (step 2-3). In addition, the inversematrix of the matrix R of the correlation coefficients is calculated.The result obtained is expressed by a matrix A (step 2–4). “Σ” in theexpression (2) indicates a summation related to a suffix i.

$\begin{matrix}{r_{pq} = {r_{qp} = \frac{\sum\;\left( {Y_{ip}Y_{iq}} \right)}{\left( {\sum\;{Y_{ip}^{2}{\sum Y_{iq}^{2}}}} \right)^{1/2}}}} & (2)\end{matrix}$Correlation Coefficient Matrix

$\begin{matrix}{R = \begin{pmatrix}1 & r_{12} & r_{13} & \cdots & r_{1\; k} \\r_{21} & 1 & r_{23} & \cdots & r_{2\; k} \\r_{31} & r_{32} & 1 & \cdots & r_{3\; k} \\\vdots & \vdots & \vdots & \vdots & \vdots \\r_{k\; 1} & r_{k\; 2} & r_{k\; 3} & \cdots & 1\end{pmatrix}} & (3)\end{matrix}$Inverse Matrix

$\begin{matrix}{A = \begin{pmatrix}a_{11} & a_{12} & a_{13} & \cdots & a_{1\; k} \\a_{21} & a_{22} & a_{23} & \cdots & a_{2\; k} \\a_{31} & a_{32} & a_{33} & \cdots & a_{3\; k} \\\vdots & \vdots & \vdots & \vdots & \vdots \\a_{k\; 1} & a_{k\; 2} & a_{k\; 3} & \cdots & a_{k\mspace{11mu} k}\end{pmatrix}} & (4)\end{matrix}$

With the calculations, the values of parameters calculated in acalculation expression, used when only the index value is calculated, isdetermined. Since all the data groups handled here express a normalstate, it is considered that the various pieces of information acquiredhave a predetermined correlation. When the current state is far from thenormal state and is likely to cause an abnormal state such as a failure,the correlations between the parameters are disturbed, and “distances”from original values (averages in a stable state) in themulti-dimensional spaces defined above increase. The distances representthe index values.

FIG. 4 is a flowchart of a procedure for calculating the index value instep 1-2 in FIG. 2. An index value at an arbitrary timing is calculatedas follows. Data x₁, x₂, . . . , x_(k) of k types in an arbitrary stateare acquired (step 3-1). The types of the data correspond to y₁₁, y₁₂, .. . , y_(1k) or the like. The information acquired is standardized usingan expression (5) (step 3-2). In this case, the standardized data aredefined as X₁, X₂, . . . , X_(k). A calculation expression (6) that isdetermined using elements a_(kk) of the inverse matrix A, is used tocalculate an index value D². A value D that is the square root of theindex value is called “Mahalanobis's distance”. “Σ” in the expression(6) indicates a summation related to the suffixes p and q.X_(i)=(x_(i)−y_(j))/σ_(j)  (5)D²=(1/k)Σa_(pq)X_(p)X_(q)  (6)

The process of determining the calculation method for the index valuesincludes determining the calculation expression for the index values andcalculating the index value D using the calculation expression to updatethe index value D. This process may be continuously executed while theimage forming system 6 is operated. In such case, the flow chart of theprocesses is obtained by combining the steps in FIG. 2 and those in FIG.3.

FIG. 6 illustrates a configuration of a color copying machine accordingto an embodiment of the present invention. The color copying machineserves as an image forming apparatus that uses an electronicphotographing scheme. The image forming system 6 (see FIG. 1) serving asthe image forming unit of the color copying machine includes a printerunit 100, a paper feeding unit 200, a scanner unit 300, and a originalconvey unit 400. The scanner unit 300 is fixed on the copying machinemain body. An original convey unit 400 constituted by an originalautomatic convey device (ADF) is fixed on the scanner unit 300. Inaddition, the copying machine main body also includes the control unit 5(see FIG. 1) that controls the operations of the various devices in thecolor copying machine. The control unit 5 includes a CPU, a RAM, a ROM,an I/O interface unit, and the like as described above.

In the scanner unit 300, a read sensor 36 reads the image information ofan original document placed on a contact glass 32 and transmits theimage information read to the control unit. A laser, an LED, or the like(not shown) is arranged in an exposure device 21 in the printer unit100. The control unit controls the laser, the LED or the like, on thebasis of the image information received from the scanner unit 300, toirradiate a laser write beam L on photosensitive drums 40Bk, 40Y, 40M,and 40C. With this irradiation, electrostatic latent images are formedon the surfaces of the photosensitive drums 40Bk, 40Y, 40M, and 40C, andthe latent images are developed into toner images by a predetermineddeveloping process.

In addition to the exposure device 21, the printer unit 100 includes aprimary transfer device 62, a secondary transfer device 22, a fixingdevice 25, a delivery device, a toner supply device (not shown), and thelike. The developing process will be described later in detail.

The paper feeding unit 200 includes paper feeding cassettes 44 held in aplurality of levels in a paper bank 43, a paper feeding roller 42 thatforwards transfer paper P, which serves as a recording medium, from apaper feeding cassette, a separation roller 45 that separates thetransfer paper P forwarded to send the transfer paper P along a paperfeeding path 46, a convey roller 47 that conveys the transfer paper P toa feeding path 48 of the printer unit 100, and the like. In theapparatus according to the embodiment, in addition to the paper feedingunit 200, a manual paper feeding tray 51 that is used for feeding papermanually and a separation roller 52 that separates sheets of transferpaper P on the manual paper feeding tray one by one toward a manualpaper feeding path 53, are arranged on a side surface of the apparatus.A resist roller 49 delivers only one sheet of transfer paper P placed onthe paper feeding cassette 44 or the manual paper feeding tray 51, andsends the sheet of transfer paper to a secondary transfer nip portionlocated between an intermediate transfer belt 10 serving as anintermediate transfer body and the secondary transfer device 22.

In the configuration, when a color image is to be copied, an originaldocument is set on an original table 30 of the original convey unit 400.Alternatively, the original convey unit 400 is opened to set theoriginal document on the contact glass 32 of the scanner unit 300, andthe original convey unit 400 is closed to press the original document.When a start switch (not shown) is pressed, the original document isconveyed onto the contact glass 32 if the original document is set onthe original convey unit 400. On the other hand, if the originaldocument is set on the contact glass 32, the scanner unit 300 isimmediately driven to cause a first moving body 33 and a second movingbody 34 to move. The first traveling body 33 reflects a beam from alight source, and a reflected beam from the surface of the originaldocument is reflected to the second traveling body 34. The mirror of thesecond traveling body 34 reflects the beam and inputs the beam to theread sensor 36 through the image forming lens 35 to read imageinformation. When the image information is received from the scannerunit, the laser writing and a developing process (to be described later)is performed to form toner images on the photosensitive drums 40Bk, 40Y,40M, and 40C. At the same time, one of four resist-rollers is operatedto feed the transfer paper P of a size depending on the imageinformation read.

Accordingly, a drive motor (not shown) rotationally drives one of thesupport rollers 14, 15, and 16, that in turn rotate other two supportrollers, and the intermediate transfer belt 10 is conveyed by therotation of these rollers. At the same time, image forming units 18rotate the photosensitive drums 40Bk, 40Y, 40M, and 40C to formsingle-color images of black, yellow, magenta, and cyan on thephotosensitive drums 40Bk, 40Y, 40M, and 40C, respectively. With theconveyance of the intermediate transfer belt 10, these single-colorimages are sequentially transferred to form a synthesized color image onthe intermediate transfer belt 10.

On the other hand, one of the paper feeding rollers 42 of the paperfeeding unit 200 is selectively rotated to forward sheets of transferpaper P from one of the paper feeding cassettes 44. The separationroller 45 separates the sheets of transfer paper P and puts one sheet oftransfer paper P at a time, into the feeding path 46. The convey roller47 guides the sheet of transfer paper P to the feeding path 48 in theprinter unit 100 and hits the sheet of transfer paper P against theresist roller 49 to stop the sheet of paper. Alternatively, a paperfeeding roller 50 is rotated to forward sheets of transfer paper P onthe manual paper feeding tray 51, separated by the separation roller 52,put into the manual paper feeding path 53, and hit against the resistroller 49 to stop the sheet of transfer paper P. The resist roller 49 isrotated at a timing matched with the timing when the synthesized colorimage is transferred on the intermediate transfer belt 10, and the sheetof transfer paper P is sent to the secondary nip portion that serves asa contact unit between the intermediate transfer belt and a secondarytransfer roller 23. The color image is transferred by the influence of atransfer electric field and a contact pressure generated at the nip torecord the color image on the sheet of transfer paper P.

The sheet of transfer paper P with the image is sent to the fixingdevice 25 over a convey belt 24 of the secondary transfer device. In thefixing device 25, a pressing roller 27 applies pressure and heat to thetoner image to fix the toner image, and a delivery roller 56 deliversthe sheet of transfer paper P onto a paper delivery tray 57.

The details of the printer unit 100 in the color copying machineaccording to the embodiment will be described below.

FIG. 7 is an enlarged view of a main part of the printer unit 100. Theprinter unit 100 includes an intermediate transfer belt 10 supported bythree support rollers 14, 15, and 16, four photosensitive drums 40Bk,40Y, 40M, and 40C serving as latent image carriers each of which carriesa toner image of one of black, yellow, magenta, and cyan, and developingunits 61Bk, 61Y, 61M, and 61C serving as developing units that formtoner images on the drum surfaces. The printer unit 100 also includesphotosensitive body cleaning devices 63Bk, 63Y, 63M, and 63C. Four imageforming units 18Bk, 18Y, 18M, and 18C include the photosensitive drums40Bk, 40Y, 40M, and 40C, the developing units 61Bk, 61Y, 61M, and 61C,and the photosensitive body cleaning devices 63Bk, 63Y, 63M, and 63C,respectively, and constitute a tandem image forming device 20. A beltcleaning device 17 removes residual toner remaining on the intermediatetransfer belt 10 after a toner image is transferred to a sheet oftransfer paper, and is arranged on the left of the support roller 15 inFIG. 7.

The belt cleaning device 17 has two fur brushes 90 and 91 as cleansingmembers. The fur brushes 90 and 91 (φ20 millimeters) are made of acryliccarbon (6.25 D/F), having a resistance of ×10⁷, and planted at 0.1million/inch². The fur brushes 90 and 91 are arranged to be into contactwith the intermediate transfer belt 10 and rotated. A power supply (notshown) applies biases having different polarities to the fur brushes 90and 91. Metal rollers 92 and 93 are brought into contact with the furbrushes 90 and 91, respectively, to make it possible to rotate the metalrollers 92 and 93 in a forward or backward direction with respect to thefur brushes.

In the embodiment, a power supply 94 applies a negative voltage to themetal roller 92 on the upstream side of the intermediate transfer belt10 in the rotating direction, and a power supply 95 applies a positivevoltage to the metal roller 931 on the downstream side. The distal endsof blades 96 and 97 are brought into press contact with the metalrollers 92 and 93, respectively. When the intermediate transfer belt 10rotates in the direction shown by an arrow in FIG. 7, the fur brush 90on the upstream side is used first, to apply, for example, a negativebias, to thereby clean the surface of the intermediate transfer belt 10.If a voltage of −700 volts is applied to the metal roller 92, the furbrush 90 has a voltage of −400 volts, and positively charged toner onthe intermediate transfer belt 10 can be transferred to the side of thefur brush 90. The toner transferred to the fur brush side is transferredfrom the fur brush 90 to the metal roller 92 by a potential difference,and the blade 96 scrapes out the toner.

In this manner, the fur brush 90 removes the toner on the intermediatetransfer belt 10. However, a large amount of toner still remains on theintermediate transfer belt 10. The toner is negatively charged by anegative bias applied by the fur brush 90. It is considered that thetoner is charged by injection of electric charge or discharge.Therefore, a positive bias is applied next, using the fur brush 91, toclean the intermediate transfer belt 10, to remove the remaining toner.The removed toner is transferred from the fur brush 91 to the metalroller 93 by a potential difference, and the toner is scraped out by theblade 97. The toner scraped out by the blades 96 and 97 is recovered andput in a tank (not shown). The toner may be returned to the developingdevice 61 by using a toner recycle device (to be described later).

Thus, as described above, although most of the toner is removed from thesurface of the intermediate transfer belt 10 by cleaning with the furbrush 91, a small amount of toner still remains on the surface. Thetoner remaining on the intermediate transfer belt 10 is positivelycharged by a positive bias applied to the fur brush 91. The positivelycharged toner is transferred to the photosensitive drums 40Bk, 40Y, 40M,and 40C by a transfer electric field applied at a primary transferposition, and can be recovered by the photosensitive body cleaningdevice 63.

A secondary transfer device 22 and the tandem image forming device 20are arranged on the opposite sides of the intermediate transfer belt 10.The secondary transfer device 22 is constituted such that, in theembodiment, the convey belt 24 is booked between the two rollers 23. Thesecondary transfer device 22 is brought into press contact with thethird support roller 16 through the intermediate transfer belt 10 toform a secondary transfer nip portion, and a color toner image on theintermediate transfer belt 10 is secondarily transferred onto a sheet oftransfer paper P. After the secondary transfer, the residual toner onthe intermediate transfer belt 10 is removed by the belt cleaning device17. The intermediate transfer belt 10 prepares for the next imageformation. The secondary transfer device 22 also includes function tocarry a sheet of transfer paper P, on which the image is transferred, tothe fixing device 25. A transfer roller or a non-contact charger may bearranged as the secondary transfer device 22. However, in such a case,it is difficult for the secondary transfer device 22 to execute afunction of carrying the transfer paper P.

Commonly, the resist roller 49 with earthing is used. However, a biascan also be applied to remove paper powder from the transfer paper P.For example, the bias may be applied via a conductive rubber roller.Conductive NBR rubber having a diameter of φ18 millimeters and a surfacethickness of 1 millimeter is used as the material of the rubber roller.An electric resistance is a volume resistance of the rubber material,i.e., about 10×10⁹Ω·cm, and an application voltage of about −800 voltsis applied to a side (front surface side) to which toner is transferred.A voltage of +200 volts is applied to the rear surface of the paper.

In a general intermediate transfer system, paper powder does not easilymove to a photosensitive drum. Therefore, the necessity of consideringtransfer of paper powder is less, and the photosensitive drum may begrounded. ADC bias is applied as the application voltage. However, an ACvoltage having a DC offset may be used to charge the sheet of transferpaper P more uniformly. The paper surface applied with the bias andpassing through the resist roller 49 is slightly negatively charged.Therefore, in transfer from the intermediate transfer belt 10 to thesheet of transfer paper P, the transfer conditions are different fromthose set when no voltage is applied to the resist roller 49, and thetransfer conditions may be changed.

In the embodiment, a transfer paper reversing device 28 (see FIG. 6)that is arranged in parallel to the tandem image forming device 20,reverses the sheet of transfer paper P to record images on both thesurfaces of the sheet of transfer paper P. In this manner, after theimage is fixed on one surface of the sheet of transfer paper, the courseof the sheet of transfer paper is switched to the transfer paperreversing device side by a switching pawl. At this position, the sheetof transfer paper is reversed, and the toner image is transferred by thesecondary transfer nip again. Thereafter, the sheet of transfer paper Pmay be delivered on the paper delivery tray.

The tandem image forming device 20 will be described below.

FIG. 8 is a partially enlarged view of the tandem image forming device20. The four image forming units 18Bk, 18Y, 18M, and 18C have identicalconfigurations, and hence, the four color symbols Bk, Y, M and C areomitted in the description that follows. The configuration of one of theunits will be described in detail. As shown in FIG. 8, in the imageforming unit, a charging device 60, a developing device 61, a primarytransfer device 62, a photosensitive body cleaning device 63, an ionizer64, and the like are arranged around the photosensitive drums 40Bk, 40Y,40M, and 40C. Each of the photosensitive drums 40Bk, 40Y, 40M, and 40C,is formed by coating an organic photosensitive material on a materialtube consisting of aluminum or the like to form a photosensitive layer.Alternatively, photosensitive drums 40Bk, 40Y, 40M, and 40C may beconstituted by endless belts.

Although not shown, at least photosensitive drums 40Bk, 40Y, 40M, and40C are arranged, and a process cartridge is constituted by includingall or some of the units in the image forming unit 18. The image formingunits 18 may be detachably arranged in the printer unit 100 at once toimprove the maintenance properties. Of the units constituting the imageforming units 18, the charging device 60 is in the form of a roller inthe shown example and brought into contact with the photosensitive drums40Bk, 40Y, 40M, and 40C to apply a voltage, to charge the photosensitivedrums 40Bk, 40Y, 40M, and 40C. Alternatively, a non-contact Scorotroncharger may also be used for charging the photosensitive drums.

A one-component developing agent may be used as the developing device61. However, in the example shown, a two-component developing agentconsisting of a magnetic carrier and non-magnetic toner is used. Astirring unit 66 conveys the two-component developing agent whilestirring the two-component developing agent, supplies the two-componentdeveloping agent to a developing sleeve 65, and causes the two-componentagent to adhere to the developing sleeve 65. A developing unit 67transfers the toner of the two-component agent adhering the developingsleeve 65 to the photosensitive drums 40Bk, 40Y, 40M, and 40C. Thestirring unit 66 is located at a level lower than that of the developingunit 67.

The stirring unit 66 has two parallel screws 68. A partition plate 69partitions the two screws 68 in regions other than both the endportions. A toner concentration sensor 71 is arranged in a developingcase 70.

In the developing unit 67, the developing sleeve 65 is arranged inopposition to the photosensitive drums 40Bk, 40Y, 40M, and 40C throughan opening of the developing case 70, and a magnet 72 is fixed in thedeveloping sleeve 65. A doctor blade 73 is arranged such that the distalend of the doctor blade 73 is close to the developing sleeve 65. In theexample shown, an interval between the doctor blade 73 and thedeveloping sleeve 65 at the closest portion is set to 500 micrometers.

The developing sleeve 65 is a non-magnetic rotatable sleeve. A pluralityof magnets 72 are arranged in the developing sleeve 65. The magnet 72 isdesigned to cause magnetic force to act when the developing agent passesthrough a predetermined position. In the example shown, the diameter ofthe developing sleeve 65 is set to φ18 millimeters, and the surface ofthe developing sleeve 65 is subjected to a sandblast process or aprocess of forming a plurality of grooves each having a depth of 1 toseveral millimeters, so that a surface roughness (Rz) falls within therange of 10 to 30 micrometers.

The magnet 72 has five polarities N1, S1, N2, S2, and S3 in a directionfrom the position of the doctor blade 73 in the rotating direction ofthe developing sleeve 65. A magnetic brush, made of the developing agentand magnetized by the magnet 72, is supported on the developing sleeve65. The developing sleeve 65 is arranged in opposition to thephotosensitive drums 40Bk, 40Y, 40M, and 40C in a region on S1 side ofthe magnet 72 that forms the magnetic brush for the developing agent.

With such configuration, the two-component developing agent is conveyedand circulated while being stirred by the two screws 68, and is suppliedto the developing sleeve 65. The developing agent supplied to thedeveloping sleeve 65 is scooped up and held by the magnet 72 to form amagnetic brush on the developing sleeve 65. The magnetic brush isthinned by the doctor blade 73 to have an appropriate amount withrotation of the developing sleeve 65. The cut developing agent isreturned to the stirring unit 66.

The toner of the developing agent supported on the developing sleeve 65is transferred to the photosensitive drums 40Bk, 40Y, 40M, and 40C byapplying a developing bias voltage to the developing sleeve 65 to changethe electrostatic latent images on the photosensitive drums 40Bk, 40Y,40M, and 40C into visible images. After the visible images are formed, adeveloping agent remaining on the developing sleeve 65 is separated fromthe developing sleeve 65 out of the magnetic force of the magnet 72, andreturned to the stirring unit 66. As the operations are repeated, thetoner concentration in the stirring unit 66 decreases. The tonerconcentration sensor 71 detects the toner concentration, and toner issupplied to the stirring unit 66.

In the apparatus according to the embodiment, the following settings ofthe units are made. The linear velocity of each of the photosensitivedrums 40Bk, 40Y, 40M, and 40C is set at 200 mm/s. The linear velocity ofthe developing sleeve 65 is set at 240 mm/s. The diameter of each of thephotosensitive drums 40Bk, 40Y, 40M, and 40C is set at 50 millimeters,and the diameter of the developing sleeve 65 is set at 18 millimeters.The developing process is performed with these settings. An amount ofcharge of toner on the developing sleeve 65 preferably falls within therange of −10 to −30 μC/g. A developing gap GP, that is, each of gapsbetween the photosensitive drums 40Bk, 40Y, 40M, and 40C and thedeveloping sleeves 65 can be set to fall within the range of 0.8millimeter to 0.4 millimeter as in a conventional art. Reducing thedeveloping gap GP improves the developing efficiency. In addition, thethickness of the photosensitive body 40 is set at 30 micrometers, thebeam spot diameter of an optical system is set at 50 to 60 micrometer,and light intensity is set at 0.47 mW. A charging potential beforeexposure V0 of the photosensitive body 40 is set at −700 volts,potential after exposure VL is set at −120 volts, and a developing biaspotential is set at −470 volts, i.e., a developing potential is 350volts. A developing process is performed with these settings.

The roller-shaped primary transfer roller 62 is arranged to be in presscontact with the photosensitive body 40 through the intermediatetransfer belt 10. An electric conductive roller 74 is arranged betweenthe primary transfer devices 62 such that the electric conductive roller74 is brought into contact with a base layer U of the intermediatetransfer belt 10. The image forming units 18 are adjacent to the primarytransfer devices 62. Therefore, the electric conductive roller 74prevents biases, applied by the primary transfer devices 62 in transfer,from flowing in the image forming units 18 through the base layer Uhaving an intermediate resistance.

A cleaning blade 75 is made of polyurethane rubber. The photosensitivebody cleaning device 63 brings the distal end of the cleaning blade 75into press contact with the photosensitive body 40. In addition, a furbrush 76, having contact conductivity and an external periphery being incontact with the photosensitive body 40, is rotatably arranged in thedirection of an arrow shown in FIG. 8, to thereby improve cleaningproperties. A metal electric field roller 77 applies a bias to the furbrush 76, and is arranged such that the metal electric field roller 77can be rotated in the direction of the arrow shown. The distal end of ascraper 78 is brought into press contact with the metal electric fieldroller 77. A recovery screw 79 that recovers the removed toner is alsoarranged in the photosensitive body cleaning device 63.

In the photosensitive body cleaning device 63 with such configuration,the fur brush 76 that rotates in the direction opposite to that of thephotosensitive body 40 removes residual toner on the photosensitive body40. The electric field roller 77 that is in contact with the fur brush76, applies a bias, and rotates in the direction opposite to that of thefur brush 76, to thereby remove the toner adhering to the fur brush 76.The scraper 78 cleans the electric field roller 77 and removes the toneradhering to the electric field roller 77. The recovery screw 79 collectsthe toner, recovered by the photosensitive body cleaning device 63, onone side of the photosensitive body cleaning device 63. A toner recycledevice 80 returns the toner collected to the developing device 61, andrecycles the toner returned.

The ionizer 64 uses an ionizing lamp to irradiate a beam on thephotosensitive drum 40, to thereby initialize the surface potential ofthe photosensitive drum 40.

The image forming process, in the tandem image forming device 20 withthe above configuration, is performed as follows. With rotation of thephotosensitive drum 40, the charging device 60 uniformly charges thesurface of the photosensitive drum 40, and a write beam L is irradiatedon the photosensitive drum 40 to form an electrostatic latent image onthe photosensitive drum 40. Thereafter, the developing device 61performs developing to cause the toner to adhere to the electrostaticlatent image, and forms a toner image. The primary transfer device 62primarily transfers the toner image onto the intermediate transfer belt10. The photosensitive body cleaning device 63 removes residual tonerfrom the surface of the photosensitive drum 40 after the image transfer,and the ionizer 64 ionizes the surface to prepare image formation again.On the other hand, the residual toner removed from the surface of thephotosensitive drum is re-used in developing by a toner recycle device(to be described later). An order of colors forming an image is notlimited to the order described above. The order changes depending onobjects or characteristics held in the image forming apparatus.

The type of information to be acquired for predicting occurrence of anabnormal state in the color copying machine having the aboveconfiguration and an acquiring method will be described below.

(a) About Sensing Information

A drive relationship, various characteristics of a recording medium,characteristics of a developing agent, characteristics of aphotosensitive body, various process states of an electronic photograph,an environmental condition, various characteristics of a recordingobject, and the like are considered as the sensing information to beacquired. The outline of the pieces of sensing information will bedescribed below.

(a-1) Information of Drive

-   A rotating speed of a photosensitive drum is detected by an encoder,    a current value of a drive motor is read, and a temperature of the    drive motor is read.-   Similarly, drive states of cylindrical or belt-like rotatable units    such as a fixing roller, a paper convey roller, and a drive roller    are detected.-   A microphone installed inside or outside the apparatus detects the    sound generated by a drive.    (a-2) State of Paper Conveyance-   The positions of the front and rear ends of conveyed paper are read    by a transmissive or reflective photo-sensor or a contact type    sensor, to detect occurrence of paper jam or to read a difference    between pass timings of the front and rear ends of the sheet of    paper and a change of a direction vertical to a transmission    direction.-   Similarly, on the basis of the timings detected by the sensors, a    moving speed of the sheet of paper is calculated.-   Slit between a paper feed roller and a sheet of paper in paper    feeding is calculated by comparing a value obtained by measuring a    rotating speed of the roller with a moving distance of the sheet of    paper.    (a-3) Various Characteristics of Recording Medium Such As Paper

This information considerably affects image quality and the stability ofpaper conveyance. The information about the paper is acquired by thefollowing methods.

-   The thickness of the sheet of paper is calculated by the following    method. The sheet of paper is pinched by two rollers, relative    displacements of the roller are detected by an optical sensor, or a    displacement which is equal to a moving distance of a member lifted    up by insertion of the sheet of paper is detected.-   The surface roughness of the sheet of paper is calculated by the    following method. A guide or the like is brought into contact with    the surface of the sheet of paper before transfer, and vibration,    sliding sound, or the like generated by the contact is detected.-   The gloss of the sheet of paper is calculated by the following    method. Alight flux having a predetermined open angle is incident at    a predetermined incident angle. The light flux reflected in a    reflecting direction of a mirror surface and having a predetermined    open angle, is measured by a sensor.-   The rigidity of the sheet of paper is calculated by detecting a    transformation ratio (curvature) of the pressed sheet of paper.-   To decide whether the paper is a sheet of recycled paper, an    ultraviolet ray is irradiated on the sheet of paper and the    transmittance of the sheet of paper is detected.-   To decide whether the paper is a sheet of backing paper, a beam is    irradiated from a linear beam source such as an LED array, and a    beam reflected from a transfer surface is detected by a solid-state    image pickup element such as a CCD.-   To decide whether the paper is a sheet of paper for OHP, a beam is    irradiated on the sheet of paper to detect a regularly reflected    beam having an angle different from that of a transmitted beam.-   Moisture content of the sheet of paper is calculated by measuring    absorption of infrared or a μ-wave beam.-   A photo-sensor, a contact sensor, or the like detects an amount of    curl.-   An electric resistance of the sheet of paper is obtained by the    following method. A pair of electrodes (paper feeding rollers or the    like) is brought into contact with a sheet of recording paper to    directly measure the electric resistance. Alternatively, the surface    potential of the photosensitive body or the intermediate transfer    body after paper transfer is measured to estimate the resistance of    the sheet of recording paper, based on the surface potential.    (a-4) Developing Agent Characteristics

The characteristics of a developing agent (toner carrier) in theapparatus have a major effect on the functioning of an electronicphotographing process. Therefore, the characteristics of the developingagent serve as an important factor for an operation or an output of thesystem. It is very important to obtain the information about thedeveloping agent. The characteristics of the developing agent are givenbelow.

-   With respect to a toner, charge amounts, a distribution of charge    amounts, fluidity, a degree of agglutination, dimensional    concentration, an electric resistance, an external additive content,    a consumption of external additive or a remaining amount of external    additive, fluidity, a toner concentration (mixture ratio of toner    and a carrier) are cited.-   With respect to a carrier, magnetic characteristics, a coat    thickness, an amount of spent, and the like are cited.

It is generally difficult to independently detect the items in the imageforming apparatus. Therefore, these items are detected as integratedcharacteristics. An example method to measure the integratedcharacteristics of the developing agent is described below.

-   A test latent image is formed on a photosensitive body and developed    under predetermined developing conditions, and a reflection    concentration (optical reflectance) of the toner image formed is    measured.-   A pair of electrodes is arranged in the developing device to measure    a relationship between an application voltage and a current    (resistance, dielectric constant, or the like).-   A coil is arranged in the developing device to measure    voltage-current characteristics (inductance).-   A level sensor is arranged in the developing device to detect a    developing agent capacity. An optical level sensor, an electric    capacitance type level sensor, or the like is used as the level    sensor.    (a-5) Photosensitive Body Characteristics

Like the developing agent characteristics, the photosensitive bodycharacteristics are closely related to the functioning of an electronicphotographing process. A thickness of a photosensitive body, surfacecharacteristics (friction coefficient and unevenness), surfacepotentials (before and after the processes), surface energy, scatteringlight, a temperature, a color, a surface position (fluctuation), alinear velocity, a potential attenuation rate, a resistance/capacitance,a surface moisture content, and the like are cited as the pieces ofinformation of the photosensitive body characteristics. Of these items,the following pieces of information can be detected in the image formingapparatus.

-   A change of the electric capacitance with a change in thickness    collates with voltage-current characteristics between a detected    current flowing from a charged member to a photosensitive body and a    voltage simultaneously applied to the charged member with respect to    a dielectric thickness of a predetermined photosensitive body, to    thereby calculate a film thickness.-   The surface potential and the temperature can be calculated by a    conventionally known sensor.-   The linear velocity is detected by an encoder fixed to the rotating    shaft of the photosensitive body.-   Light scattering from the surface of the photosensitive body is    detected by a photo-sensor.    (a-6) Electronic Photographing Process State

Formation of a toner image by an electronic photographing scheme isperformed in the following order. That is, uniform charging of aphotosensitive body, latent image formation (image exposure) by a laserbeam or the like, developing by a charged toner (color particles),transfer of the toner image to a transfer material (for a color image,overlapping on a recording medium serving as an intermediate transferbody or a final transfer material or overlapping developing on thephotosensitive body in developing is performed), and fixing of the tonerimage to the recording medium. Various pieces of information about thesestages considerably affect the image and other outputs from the system.It is important to acquire the pieces of information, to evaluate thestability of the system. The following are concrete examples ofacquiring the pieces of information of the electronic photographingprocess state.

-   A conventionally known surface potential sensor detects a charge    potential and a potential of an exposing unit.-   A gap between a charged member and a photosensitive body in    non-contact charging is detected by measuring an amount of light    caused to pass through the gap.-   A wide-band antenna captures an electromagnetic wave generated by    charging.-   Sound generated by charging-   Exposure strength-   Wavelength of exposure light

The following are methods of acquiring various states of a toner image.

-   To calculate a pile height (height of a toner image), a displacement    sensor calculates a depth in the vertical direction, and a linear    sensor for parallel beams measures a light-shielding length in the    horizontal direction.-   A toner charge amount is calculated by a ratio of a potential of an    electrostatic latent image of an all-overlaying portion to an amount    of adhesion which is measured by a potential sensor that measures a    potential in a developing state of the latent image and which is    converted from a reflection concentration sensor at the same    position.-   A dot fluctuation or a dot gap is calculated by the following    method. An infrared area sensor detects a dot pattern image on a    photosensitive body. An area sensor that has wavelengths depending    on colors on an intermediate transfer body detects the dot pattern,    and an appropriate process is performed.-   To calculate an amount of offset (after fixing), the corresponding    positions on a sheet of recording paper and a fixing roller are read    out by an optical sensor and compared with each other.-   An optical sensor is installed after the transfer process (on a PD    or on a belt), and a remaining amount of transfer is decided by an    amount of reflected light from a transfer remaining pattern obtained    after a specific pattern is transferred.-   Color unevenness in overlapping is detected by a full-color sensor    that detects a fixed image on the sheet of recording paper.    (a-7) Characteristic of Toner Image Formed-   An image concentration and a color are optically detected (Any one    of a reflectance and a transmittance may be used. A projecting    wavelength is selected depending on a color). A concentration and    monochromatic information may be obtained on a photosensitive body    or an intermediate transfer body. However, a combination of colors    such as color unevenness must be measured on a sheet of paper.-   To calculate tone property, an optical sensor reflection detects    concentration of toner images formed on a photosensitive body at    tone levels or toner images transferred to a transfer body.with the    second group optical system, changes the focal length by changing    the relative spacing of the group optical systems, and in order to    focus moves the third group optical system along the optical axis.    In a variable-focal-length lens of this type, the first group    optical system is so constituted that it has a negative meniscus    lens, a negative meniscus lens, and a positive lens arranged in that    order from the object side, an aspheric surface being formed on at    least one surface among the two negative meniscus lenses; the second    group optical system is so constituted that it has a cemented lens    of a positive lens and a negative lens, a positive lens, and a    positive lens arranged in that order from the object side, forming    an aspheric surface on the surface on the object side of the    positive lens that is most on the object side; and the third group    optical system consists of a single positive lens that does not    include an aspheric surface (corresponding to claim 1).

That is, typically with this composition, in a variable-focal-lengthlens of the type described above, the first group optical system is soconstituted that it has a negative meniscus lens, a negative meniscuslens, and a positive lens arranged in that order from the object side;the second group optical system is so constituted that it has a cementedlens of a positive lens and a negative lens, a positive lens, and apositive lens arranged in that order from the object side; and the thirdgroup optical system consists of a single positive lens. In addition, anaspheric surface is formed on at least one surface of the negativemeniscus lenses in the first optical system and on the surface that ismost on the object side in the second group optical system; and thethird optical system is constituted with only a spherical lens(corresponding to claim 2).

When this is done, it becomes possible to easily make corrections inimage surface by giving the positive lens of the third group opticalsystem a meniscus shape (corresponding to claim 3).

Also, particularly satisfactory correction is possible under thecondition that−0.75<|(R1−R2)/(R1+R2)|<−0.65  (1)where R1 is the radius of curvature of the object-side surface of thepositive lens of the third group optical system, and R2 is the radius ofcurvature of the image-side surface of the positive lens of the thirdgroup optical system (corresponding to claim 4). If the lower limit ofthis condition formula (1) is not satisfied, the surrounding imagesurface will fall on the plus side, that is, in the direction away fromthe object, and if it exceeds the upper limit, the surrounding imagesurface will fall on the minus side.

Also, it is possible to shorten the minimum picture-taking distancewhile minimizing the increase in overall length, and to makesatisfactory aberration corrections, under the condition that1.5<|(D23w×f3)/fw²|<2.5  (2)where D23w is the distance between the second group optical system andthe third group optical system at the wide-angle end, fw is the focallength of the entire system at the wide-angle end, and f3 is the focallength of the third group optical system. If the upper limit of thiscondition formula (2) is exceeded, it becomes difficult to obtain a goodimage, due to an increase in the Petzval sum and an increase in thenegative distortion aberration, and if the lower limit is not satisfied,distortion correction becomes difficult because the refractive power ofthe third group optical system becomes too strong to ensure the spacingbetween the second group optical system and the third group opticalsystem (corresponding to claim 5).

If a picture-taking lens system is made using a variable-focal-lengthlens such as described above as an optical system, a picture-taking lensunit can be made that can satisfactorily correct aberrations, can have ashort minimum picture-taking distance and a wide picture-taking range,and can have a low-cost, compact configuration (corresponding to claim6).

Also, if a camera is made using a variable-focal-length lens such asdescribed above as a picture-taking optical system, a camera can be madethat can satisfactorily correct aberrations, can have a short minimumpicture-taking distance and a wide picture-taking range, and can below-cost and compact (corresponding to claim 7).

Similarly, if a portable information terminal is put together using avariable-focal-length lens such as described above as the picture-takingoptical system of a camera function unit, a portable informationterminal can be made that can satisfactorily correct aberrations, canhave a short minimum picture-taking distance and a wide picture-takingrange, and can be low-cost and compact (corresponding to claim 8).

WORKING EXAMPLES

Next, we describe in detail specific working examples, based on theabove embodiments of this invention. The first and second workingexamples discussed below are also the first and second embodiments andat the same time are working examples of specific configurationsaccording to examples of specific numerical values for thevariable-focal-length lens of this invention, and the third workingexample is an embodiment of a camera or portable information terminal ofthis invention that adopts in its picture-taking optical system thepicture-taking lens unit of this invention using a variable-focal-lengthlens such as shown in the first and second working examples.

The first and second working examples showing the variable-focal-lengthlens of this invention show the composition of the variable-focal-lengthlens and examples of its specific numerical values.

Aberration is fully corrected in the first and second working examples.It will become clear from these first and second working examples thatby putting together a variable-focal-length lens as in this invention,it will be possible to ensure very good imaging performance whileachieving satisfactory small size.

The following symbols are used in the descriptions relating to the firstand second embodiments below.

An aspheric surface is defined by the following formula, where H is theheight from the optical axis, S is the amount of displacement from theplane vertex in the direction of the optical axis, R is the radius ofcurvature, and A₂ is the asphericity.

[Numerical formula 1]

$\begin{matrix}{S = {\frac{\left( {1/R} \right){xH}^{2}}{1 + \sqrt{1 - {\left( {1/R} \right)^{2}{xH}^{2}}}} + {\sum\limits_{i}\;{A_{si}{xH}^{2i}}}}} & (3)\end{matrix}$

First Working Example

FIGS. 1, 2, and 3 show the composition of the optical system of thevariable-focal-length lens of the first working example of thisinvention at, respectively, the single-focal-length [sic; possibly amisprint for “short-focal-length”] end, that is, the wide-angle end, atthe middle focal length, and at the long-focal-length end, that is, thetelephoto end.

The variable-focal-length lens shown in FIG. 1 to FIG. 3 have first lensE1, second lens E2, third lens E3, fourth lens E4, fifth lens E5, sixthlens E6, seventh lens E7, eighth lens E8, diaphragm FA, and opticalfilter OF. In this case, the first lens E1 to the third lens E3constitute the first group optical system G1, the fourth lens E4 to theseventh lens E7 constitute the second group optical system G2, theeighth lens E8 constitutes the third group optical system G3, they aresupported by a support frame, etc. that is suitably shared by eachgroup, and the groups move as one when zooming, etc. Also, the surfacenumber of each optical surface is also shown in FIG. 1 to FIG. 3.

In FIG. 1 to FIG. 3, the components are arranged in order, from the sideof the object such as the photo subject, first lens E1, second lens E2,third lens E3, diaphragm FA, fourth lens E4, fifth lens E5, sixth lensE6, seventh lens E7, eighth lens E8, and optical filter OF, and theimage is formed on the back of the optical filter OF, which has variousoptical filtering functions.

The first lens E1 is a negative meniscus lens shaped convex on theobject side, the second lens E2 is a negative meniscus lens shapedconvex on the object side, and the third lens E3 is a positive lensconsisting of a plano-convex lens that is convex toward the object side;the first optical system G1, which consists of these lenses, first lensE1 to third lens E3, exhibits negative refractive power as a whole. Thefourth lens E4 is a positive lens made up of a biconvex lens that isstrongly convex toward the object side; for example, it is a hybridaspheric surface lens that is made up of glass lenses and forms anaspheric surface with a resin material formed on its object-sidesurface. The fifth lens E5 is a negative lens made up of a biconcavelens that is strongly concave toward the image side; these lenses,fourth lens E4 and fifth lens E5, are cemented into one, being affixedtogether so as to form a (two-ply) cemented lens. The sixth lens E6 is apositive lens made up of a biconvex lens, and the seventh lens E7 is apositive lens made up of a biconvex lens that is strongly convex towardthe image side. The second group optical system G2, which consists ofthis three-group four-lens composition of fourth lens E4 to seventh lensE7, exhibits positive refractive power as a whole. The diaphragm FA,which is arranged on the object side of the second group optical systemG2, moves as one with the second group optical system G2. The eighthlens E8 is a positive meniscus lens formed to be convex on the objectside, and the third group optical system G3, which is made up of onlythis eighth lens E8, exhibits positive refractive power.

In changing the focal length from the wide-angle end (short-focus end)to the telephoto end (long-focus end), the first group optical system G1moves toward the object so as to describe a concave locus, and thesecond group optical system G2 moves monotonically toward the object. Infocusing from infinity to a near-distance object, the third groupoptical system G3 is moved along the optical axis toward the object. Theoptical filter OF, which is made up of parallel flat plates positionedthe most on the image side, is various filters such as a crystallow-pass filter or an infrared cutoff filter. As the focal lengthchanges, the movement of the groups changes the spacing between thegroups, specifically: the spacing D12 between the surface most on theimage side of the first group optical system G1, that is, the image-sidesurface (surface number 6) of the third lens E3, and the object-sidesurface (surface number 7) of the diaphragm FA that is integral with thesecond group optical system G2; the spacing D23 between the surface moston the image side of the second group optical system G2, that is, theimage-side surface (surface number 15) of the seventh lens E7, and thesurface most on the object side of the third group optical system G3,that is, the object-side surface (surface number 16) of the eighth lensE8; and the spacing D3F between the surface most on the image side ofthe third group optical system G3, that is, the image-size surface(surface number 17) of the eighth lens E8, and the object-side surface(surface number 18) of the optical filter OF.

In this first working example, as the focal length changes from thewide-angle side to the telephoto side, other measurements change aswell, including

-   -   overall focal length: from 4.33 to 12.22 mm    -   F number: from 2.69 to 4.53    -   half face angle: from 40° to 16°.

The properties of each optical surface are as in the following table.

[Table 1]

Optical properties

Radius of Refractive Abbe Surface curvature Spacing index number  133.126 1.00 1.71300 53.94 1^(st) lens 1^(st) group  2 6.300 1.35 1.00000optical  3 15.009 1.00 1.80610 40.74 2^(nd) lens system  4* 5.101 1.501.00000  5 9.600 2.47 1.76182 26.60 3^(rd) lens  6 0.000 13.54 1.00000 7 0.000 0.80 1.00000 diaphragm  8* 5.828 0.03 1.50703 53.43 (resin)2^(nd) group  9 6.146 3.09 1.72342 37.99 4^(th) lens optical 10 −37.8031.90 1.84666 23.78 5^(th) lens system 11 5.600 0.35 1.00000 12 13.3852.03 1.48749 70.44 6^(th) lens 13 −13.385 0.10 1.00000 14 48.080 1.351.48749 70.44 7^(th) lens 15 −18.370 1.50 1.00000 16 12.227 1.75 1.5168064.20 8^(th) lens 3^(rd) group 17 77.178 2.10 1.00000 optical system 180.000 0.48 1.54892 69.13 Filter 19 0.000 0.34 1.54892 69.13 20 0.0000.50 1.50000 64.00 21 0.000

The optical surfaces of surfaces 4 and 8 whose surface number is markedwith an asterisk (*) in Table 1 are aspheric surfaces, and theparameters of each aspheric surface in formula (3) are as in thefollowing table.

[Table 2] Asphericities

Surface 4 8 Asphericity A₄ −1.07237E−03 −4.94747E−04 A₆ −2.62545E−05−1.37113E−05 A₈ −1.39674E−08 8.40768E−07 A₁₀ −4.22743E−08 −2.81080E−08A₁₈ −3.64892E−14 0.

The spacing D12 between the first group optical system G1 and thediaphragm FA, which is integral with the second group optical system G2,the spacing D23 between the second group optical system G2 and the thirdgroup optical system G3, and the spacing D3F between the third groupoptical system G3 and the optical filter OF change as in the followingtable when the focal length changes.

[Table 3] Variable spacing

TABLE 3 Variable spacing Surfaces Wide-angle end Middle Telephoto endD12 13.54 5.56 1.20 D23 1.50 6.30 15.87 D3F 3.87 3.67 1.75

Also, the following are the numerical values of |(R1−R2)/(R1+R2)| incondition formula (1) and of |(D23w×f3)/fw²| in condition formula (2)referred to above in this first working example.

Condition formula numerical values:

-   -   (1) formula=−0.726    -   (2) formula=2.23

Therefore the numerical values for the condition formulas of thisinvention referred to above in this first working example all lie withinthe range of the condition formulas.

FIG. 4 to FIG. 6 are aberration curve diagrams for the aberrations ofthe variable-focal-length lenses shown in FIG. 1 to FIG. 3 of the abovefirst working example; FIG. 4 is an aberration curve diagram at thewide-angle end, FIG. 5 is an aberration curve diagram in the middlefocal length, and FIG. 6 is an aberration curve diagram at the telephotoend. In each aberration curve diagram, in the spherical surfaceaberration diagram the dotted line represents the sine condition, and inthe stigmatic diagram the solid line represents the saggital, and thedotted line represents the meridional.

According to these aberration curve diagrams in FIG. 4 to FIG. 6, it isclear that aberration is satisfactorily corrected or kept in check by avariable-focal-length lens of the compositions shown in FIG. 1 to FIG. 3relating to the first working example of this invention.

If done in this way, with a negative-positive-positive three-groupvariable-focal-length lens it is possible to make a picture-taking lensthat can satisfactorily correct aberrations, can have a short minimumpicture-taking distance and a wide picture-taking range, and is compact.And because the third group optical system, which formerly used anaspheric surface lens, can be made with a spherical lens, there is thefurther advantage of keeping down the cost of manufacturing it.

Second Working Example

FIG. 7, FIG. 8, and FIG. 9 show, respectively, the composition of theoptical system of the variable-focal-length lens of the second workingexample (which is also the second embodiment) of this invention at,respectively, the single-focal-length [sic; possibly a misprint for“short-focal-length”] end, that is, the wide-angle end, at the middlefocal length, and at the long-focal-length end, that is, the telephotoend.

The variable-focal-length lens shown in FIG. 7 to FIG. 9 have first lensE1, second lens E2, third lens E3, fourth lens E4, fifth lens E5, sixthlens E6, seventh lens E7, eighth lens E8, diaphragm FA, and opticalfilter OF. In this case, the first lens E1 to the third lens E3constitute the first group optical system G1, the fourth lens E4 to theseventh lens E7 constitute the second group optical system G2, theeighth lens E8 constitutes the third group optical system G3, they aresupported by a support frame, etc. that is suitably shared by eachgroup, and the groups move as one when zooming, etc. Also, the surfacenumber of each optical surface is also shown in FIG. 7 to FIG. 9. And inorder to avoid a more complicated explanation due to a larger number ofdigits for the reference symbols, the reference numbers for FIG. 7 toFIG. 9 are used independently for each working example, so even ifcommon reference numbers are assigned in FIG. 7 to FIG. 9 and in FIG. 1to FIG. 3, they do not necessarily represent a common composition withthe first working example.

In FIG. 7 to FIG. 9, the components are arranged in order, from the sideof the object such as the photo subject, first lens E1, second lens E2,third lens E3, diaphragm FA, fourth lens E4, fifth lens E5, sixth lensE6, seventh lens E7, eighth lens E8, and optical filter OF, and theimage is formed on the back of the optical filter OF, which has variousoptical filtering functions.

The first lens E1 is a negative meniscus lens shaped convex on theobject side, the second lens E2 is a negative meniscus lens shapedconvex on the object side, and the third lens E3 is a positive lensconsisting of a plano-convex lens that is convex toward the object side;the first optical system G1, which consists of these lenses, first lensE1 to third lens E3, exhibits negative refractive power as a whole. Thefourth lens E4 is a positive lens made up of a biconvex lens that isstrongly convex toward the object side; in this case too, for example,it is a hybrid aspheric surface lens that is made up of glass lenses andforms an aspheric surface with a resin material formed on itsobject-side surface. The fifth lens E5 is a negative lens made up of abiconcave lens that is strongly concave toward the image side; theselenses, fourth lens E4 and fifth lens E5, are cemented into one, beingaffixed together so as to form a (two-ply) cemented lens.

The sixth lens E6 is a positive lens made up of a biconvex lens, and theseventh lens E7 is a positive lens made up of a biconvex lens that isstrongly convex toward the image side. The second group optical systemG2, which consists of this three-group four-lens composition of fourthlens E4 to seventh lens E7, exhibits positive refractive power as awhole. The diaphragm FA, which is arranged on the object side of thesecond group optical system G2, moves as one with the second groupoptical system G2. The eighth lens E8 is a positive meniscus lens formedto be convex on the object side, and the third group optical system G3,which is made up of only this eighth lens E8, exhibits positiverefractive power.

In changing the focal length from the wide-angle end to the telephotoend, the first group optical system G1 moves toward the object side soas to describe a concave locus, and the second group optical system G2moves monotonically toward the object. In focusing from infinity to anear-distance object, the third group optical system G3 is moved alongthe optical axis toward the object. The optical filter OF, which is madeup of parallel flat plates positioned the most on the image side, isvarious filters such as a crystal low-pass filter or an infrared cutofffilter. As the focal length changes, the movement of the groups changesthe spacing between the groups, specifically: the spacing D12 betweenthe surface most on the image side of the first group optical system G1,that is, the image-side surface (surface number 6) of the third lens E3,and the object-side surface (surface number 7) of the diaphragm FA thatis integral with the second group optical system G2; the spacing D23between the surface most on the image side of the second group opticalsystem G2, that is, the image-side surface (surface number 15) of theseventh lens E7, and the surface most on the object side of the thirdgroup optical system G3, that is, the object-side surface (surfacenumber 16) of the eighth lens E8 and the spacing D3F between the surfacemost on the image side of the third group optical system G3, that is,the image-size surface (surface number 17) of the eighth lens E8, andthe object-side surface (surface number 18) of the optical filter OF.

In this second working example, as the focal length changes from thewide-angle side to the telephoto side, other measurements change aswell, including

-   -   overall focal length: from 4.33 to 12.22 mm    -   F number: from 2.64 to 4.46    -   half face angle: from 40° to 16°.

The properties of each optical surface are as in the following table.

[Table 4] Optical properties

Radius of Refractive Abbe Surface curvature Spacing index number  148.743 1.00 1.71300 53.94 1^(st) lens 1^(st) group  2 7.022 0.99 1.00000optical  3 13.235 1.00 1.80610 40.74 2^(nd) lens system  4* 4.906 1.701.00000  5 9.973 2.41 1.76182 26.60 3^(rd) lens  6 0.000 13.83 1.00000 7 0.000 0.80 1.00000 diaphragm  8* 6.052 0.02 1.50703 53.43 (resin)2^(nd) group  9 5.692 3.12 1.72342 37.99 4^(th) lens optical 10 −29.7371.93 1.84666 23.78 5^(th) lens system 11 5.657 0.32 1.00000 12 14.1481.95 1.48749 70.44 6^(th) lens 13 −14.148 0.30 1.00000 14 41.652 1.321.48749 70.44 7^(th) lens 15 −18.843 1.50 1.00000 16 10.509 1.89 1.5168064.20 8^(th) lens 3^(rd) group 17 58.182 2.14 1.00000 optical system 180.000 0.48 1.54892 69.13 Filter 19 0.000 0.34 1.54892 69.13 20 0.0000.50 1.50000 64.00 21 0.000

The optical surfaces of surfaces 4 and 8 whose surface number is markedwith an asterisk (*) in Table 4 are aspheric surfaces, and theparameters of each aspheric surface in formula (3) are as in thefollowing table.

[Table 5] Asphericities

Surface 4 8 Asphericity A₄ −1.17507E−03 −4.43095E−04 A₆ −3.05276E−05−1.70593E−05 A₈ 2.02661E−06 1.87848E−06 A₁₀ −3.84638E−07 −1.33481E−07A₁₂ 2.31751E−08 0. A₁₄ −8.01568E−10 0. A₁₆ 1.52884E−11 0. A₁₈−2.98849E−13 0.

The spacing D12 between the first group optical system G1 and thediaphragm FA, which is integral with the second group optical system G2,the spacing D23 between the second group optical system G2 and the thirdgroup optical system G3, and the spacing D3F between the third groupoptical system G3 and the optical filter OF change as in the followingtable when the focal length changes.

[Table 6] Variable spacing

TABLE 6 Variable spacing Surfaces Wide-angle end Middle Telephoto endD12 13.54 5.63 1.20 D23 1.50 6.77 15.81 D3F 3.51 3.19 1.79

Also, the following are the numerical values of |(R1−R2)/(R1+R2)| incondition formula (1) and of |(D23w×f3)/fw²| in condition formula (2)referred to above in this second working example.

Condition formula numerical values:

-   -   (1) formula=−0.694    -   (2) formula=1.96

Therefore the numerical values for the condition formulas of thisinvention referred to above in this second working example all liewithin the range of the condition formulas.

FIG. 10 to FIG. 12 are aberration curve diagrams for the aberrations ofthe variable-focal-length lenses shown in FIG. 7 to FIG. 9 of the abovesecond working example; FIG. 10 is an aberration curve diagram at thewide-angle end, FIG. 11 is an aberration curve diagram in the middlefocal length, and FIG. 12 is an aberration curve diagram at thetelephoto end.

According to these aberration curve diagrams in FIG. 10 to FIG. 12, itis clear that aberration is satisfactorily corrected or kept in checkalso by a variable-focal-length lens of the compositions shown in FIG. 7to FIG. 9 relating to the second working example of this invention.

Third embodiment

Next, we describe, with reference to FIG. 13, a third embodiment of thisinvention that comprises a camera adopting as the picture-taking opticalsystem a picture-taking lens unit made using as a zoom lens thevariable-focal-length lens of this invention such as is shown in theabove first and second working examples. FIG. 13 is a perspective viewshowing the appearance of the cameras as seen from its rear side, whichis the side of the photographer. Also, here we describe a camera, but inrecent years products have appeared in which camera functions are builtinto a so-called PDA (personal data assistant), portable telephone, orother portable information terminal. Such portable informationterminals, although they may look somewhat different, includeessentially exactly the same functions and features as a camera, and onemay adopt the variable-focal-length lens of this invention in such aportable information terminal.

As shown in FIG. 13, the camera has a picture-taking lens unit 101, ashutter button 102, a zoom button 103, an optical viewfinder 104, aliquid crystal display unit 105, a liquid crystal monitor 106, and amain switch 107.

The camera has the picture-taking lens unit 101, a CCD (charge-coupleddevice) imaging element, or other light-sensing element (not pictured)as an area sensor, and is constituted so that the image of the object tobe imaged that is formed by the picture-taking lens unit 101, which isthe picture-taking optical system, that is, the image of the photosubject, is read by the light-sensing element. Used as thispicture-taking lens unit 101 is a variable-focal-length lens of thisinvention such as is described in the first and second working examples.

The output of the light-sensing element is processed by a signalprocessor (not pictured) that is controlled by a central processing unit(CPU) (not pictured) and is converted into digital image information.The image information, digitized by the signal processor, undergoes theprescribed image processing in an image processor (not pictured) that islikewise controlled by the central processing unit, then is recorded ina nonvolatile memory or other semiconductor memory (not pictured). Inthis case, the semiconductor memory may be a memory card that is mountedin a memory card slot, etc., or it may be a semiconductor memory that isbuilt into the main body of the camera. The image being photographed canbe displayed as an electronic viewfinder on the liquid crystal monitor106, and images recorded in the semiconductor memory may also bedisplayed. Also, images recorded in the semiconductor memory can betransmitted to the outside via a communication card, etc. mounted in acommunication card slot, etc.

The picture-taking lens unit 101 is constructed so that it is heldwithin the body of the camera in retracted state when the camera isbeing carried around, and when the user operates the main switch 107 andturns on the power, the lens barrel is extended as pictured, and itprotrudes from the body of the camera. At this time, inside the lensbarrel of the picture-taking lens unit, the optical system of the groupsthat constitute the variable-focal-length lens are arranged for exampleon the short-focus end, and by operating a zoom button 103, thearrangement of the group optical systems can be changed, changing themagnification to the long-focus end. Preferably, the optical viewfinder104 also changes its magnification coupled to the change in the fieldangle of the picture-taking lens unit 101. In many cases, focusing isdone by half-pressing the shutter button 102. In this case, focusing inthe variable-focal-length lens made up of three groups,negative-positive-positive in this invention (the variable-focal-lengthlens defined in claim 1 to claim 5, or shown in the first and secondworking examples) can be done by moving the third group optical systemG3. Pressing the shutter button 102 in all the way causes a picture tobe taken, after which processing is done as described above.

As has already been stated, a variable-focal-length lens such as shownin the first and second working examples can be used as a picture-takingoptical system in a camera or portable information terminal such asdescribed above. Therefore it is possible to create a small-size cameraor portable information terminal with high picture quality usinglight-sensing elements in the 3-5 megapixel class or more. In this case,with this portable information terminal one can makehigh-picture-quality images and transmit them to outside.

Effects of the Invention

As stated above, this invention makes it possible to provide—in avariable-focal-length lens that has a first group optical system havingnegative refractive power, a second group optical system having positiverefractive power, and a third group optical system having positiverefractive power arranged in that order from the object side, has on theobject side of the second group optical system a diaphragm that moves asone with the second group optical system, and changes the focal lengthby changing the relative spacing of the group optical systems—avariable-focal-length lens that is small, has a wide field angle, andcan have high picture quality, and in addition is low-cost and can focusby moving the third group optical system along the optical axis, as wellas a lens unit, camera, and portable information terminal.

That is, according to the variable-focal-length lens of a first aspectof this invention, it is a variable-focal-length lens that has a firstgroup optical system having negative refractive power, a second groupoptical system having positive refractive power, and a third groupoptical system having positive refractive power arranged in that orderfrom the object side, has on the object side of the second group opticalsystem a diaphragm that moves as one with the second group opticalsystem, changes the focal length by changing the relative spacing of thegroup optical systems, and in order to focus moves the third groupoptical system along the optical axis; the first group optical systemhas a negative meniscus lens, a negative meniscus lens, and a positivelens arranged in that order from the object side, an aspheric surfacebeing formed on at least one surface among the two negative meniscuslenses; the second group optical system has a cemented lens of apositive lens and a negative lens, a positive lens, and a positive lensarranged in that order from the object side, forming an aspheric surfaceon the surface on the object side of the positive lens that is most onthe object side; and the third group optical system consists of a singlepositive lens that does not include an aspheric surface; and thereby inparticular it can satisfactorily correct aberrations, has a shortminimum picture-taking distance and a wide picture-taking range, and canreduce aspheric surfaces, and have a low-cost, compact configuration.

Also, according to the variable-focal-length lens of a second aspect ofthis invention, it is a variable-focal-length lens that has a firstgroup optical system having negative refractive power, a second groupoptical system having positive refractive power, and a third groupoptical system having positive refractive power arranged in that orderfrom the object side, has on the object side of the second group opticalsystem a diaphragm that moves as one with the second group opticalsystem, changes the focal length by changing the relative spacing of thegroup optical systems, and in order to focus moves the third groupoptical system along the optical axis; the first group optical systemhas a negative meniscus lens, a negative meniscus lens, and a positivelens arranged in that order from the object side; the second groupoptical system has a cemented lens of a positive lens and a negativelens, a positive lens, and a positive lens arranged in that order fromthe object side; the third group optical system consists of a singlepositive lens; an aspheric surface is formed on at least one surface ofthe negative meniscus lenses in the first optical system and on thesurface that is most on the object side in the second group opticalsystem; and the third optical system is constituted with only aspherical lens; and thereby in particular it can satisfactorily correctaberrations, has a short minimum picture-taking distance and a widepicture-taking range, and can effectively adopt spherical surfaces andhave a low-cost, compact configuration.

According to the variable-focal-length lens of a third aspect of thisinvention, it is a variable-focal-length lens of the first or secondaspect of this invention, and by the positive lens of the third opticalsystem being a positive meniscus lens, in particular it is easy to makecorrections to the image surface.

According to the variable-focal-length lens of a fourth aspect of thisinvention, it is a variable-focal-length lens of the third aspect ofthis invention, and by satisfying the condition formula:−0.75<|(R1−R2)/(R1+R2)|<−0.65  (1)where R1 is the radius of curvature of the object-side surface of thepositive lens of the third group optical system, and R2 is the radius ofcurvature of the image-side surface of the positive lens of the thirdgroup optical system, in particular better correction is possible.

According to the variable-focal-length lens of a fifth aspect of thisinvention, it is a variable-focal-length lens of any of the first tofourth aspect of this invention, and by satisfying the conditionformula:1.5<|(D23w×f3)/fw²|<2.5  (2)where D23w is the distance between the second group optical system andthe third group optical system at the wide-angle end, fw is the focallength of the entire system at the wide-angle end, and f3 is the focallength of the third group optical system, in particular the minimumpicture-taking distance can be shortened while minimizing the increasein overall length, and distortion aberration and other aberrations canbe satisfactorily corrected.

According to the variable-focal-length lens of a sixth aspect of thisinvention, by a variable-focal-length lens of any of the first to fifthaspect of this invention, in particular aberrations can be corrected,the minimum picture-taking distance can be made short and thepicture-taking range can be made wide, and a contact configuration canbe made at low cost.

According to the camera of a seventh aspect of this invention, byincluding as its picture-taking optical system a variable-focal-lengthlens of any of the first to fifth aspect of this invention, it ispossible to satisfactorily correct aberrations, have a short minimumpicture-taking distance and a wide picture-taking range, and have acompact configuration at low cost.

According to the portable information terminal of an eighth aspect ofthis invention, by including as the picture-taking optical system of itscamera function unit a variable-focal-length lens of any of the first tofifth aspect of this invention, in particular it is possible tosatisfactorily correct aberrations, have a short minimum picture-takingdistance and a wide picture-taking range, and have a compactconfiguration at low cost.

1. A variable focal length lens comprising: a first group optical systemhaving a negative refracting power, a second group optical system havinga positive refracting power, and a third group optical system having apositive refracting power, wherein the first through the third groupoptical systems are sequentially arranged from an object side; and astop provided on the object side of the second group optical system andthat moves integrally with the second group optical system, wherein afocal length is changed by changing distances between the first throughthe third group optical systems and when performing focusing the thirdgroup optical system is moved along an optical axis, the first groupoptical system includes a negative meniscus lens, a negative meniscuslens, and a positive lens those are sequentially arranged from theobject side, at least one surface of the two negative meniscus lensesbeing an aspherical surface, the second group optical system includes acemented lens of a positive lens and a negative lens, a positive lens,and a positive lens those are sequentially arranged from the objectside, a surface on the object side of the positive lens on the mostobject side being an aspherical surface, and the third group opticalsystem includes one positive lens not including an aspherical surface.2. The variable focal length lens according to claim 1, wherein thepositive lens of the third group optical system is a positive meniscuslens.
 3. The variable focal length lens according to claim 2, whereinwhen R1 is a radius of curvature a surface on an object side of thepositive lens of the third group optical system, and R2 is a radius ofcurvature of a surface on an image side of the positive lens of thethird group optical system, then the relation−0.75<{(R1−R2)/(R1+R2)}<−0.65 holds true.
 4. The variable focal lengthlens according to claim 1, wherein when D23w is a distance between thesecond group optical system and the third group optical system at thewide-angle end, fw is a focal length of all the systems at thewide-angle end, and f3 is a focal length of the third group opticalsystem, then the relation1.5<{(D23w×f3)/fw²}<2.5 holds true.
 5. A variable focal length lenscomprising: a first group optical system having a negative refractingpower, a second group optical system having a positive refracting power,and a third group optical system having a positive refracting power,wherein the first through the third group optical systems aresequentially arranged from an object side; and a stop provided on theobject side of the second group optical system and that moves integrallywith the second group optical system, wherein a focal length is changedby changing relative distances between the first through the third groupoptical systems and when performing focusing the third group opticalsystem is moved along an optical axis, the first group optical systemincludes a negative meniscus lens, a negative meniscus lens, and apositive lens those are sequentially arranged from the object side, thesecond group optical system includes a cemented lens of a positive lensand a negative lens, a positive lens, and a positive lens those aresequentially arranged from the object side, the third group opticalsystem includes one positive lens, at least one surface of the negativemeniscus lens in the first group optical system and a surface on themost object side in the second group optical system being asphericalsurfaces, and the third group optical system includes only a sphericallens.
 6. The variable focal length lens according to claim 5, whereinthe positive lens of the third group optical system is a positivemeniscus lens.
 7. The variable focal length lens according to claim 6,wherein when R1 is a radius of curvature a surface on an object side ofthe positive lens of the third group optical system, and R2 is a radiusof curvature of a surface on an image side of the positive lens of thethird group optical system, then the relation−0.75<{(R1−R2)/(R1+R2)}<−0.65 holds true.
 8. The variable focal lengthlens according to claim 5, wherein when D23w is a distance between thesecond group optical system and the third group optical system at thewide-angle end, fw is a focal length of all the systems at thewide-angle end, and f3 is a focal length of the third group opticalsystem, then the relation1.5<{(D23w×f3)/fw²}<2.5 holds true.
 9. A photographing lens unitcomprising a variable focal length lens as an optical system, thevariable focal length lens including a first group optical system havinga negative refracting power, a second group optical system having apositive refracting power, and a third group optical system having apositive refracting power, wherein the first through the third groupoptical systems are sequentially arranged from an object side; and astop provided on the object side of the second group optical system andthat moves integrally with the second group optical system, wherein afocal length is changed by changing distances between the first throughthe third group optical systems and when performing focusing the thirdgroup optical system is moved along an optical axis, the first groupoptical system includes a negative meniscus lens, a negative meniscuslens, and a positive lens those are sequentially arranged from theobject side, at least one surface of the two negative meniscus lensesbeing an aspherical surface, the second group optical system includes acemented lens of a positive lens and a negative lens, a positive lens,and a positive lens those are sequentially arranged from the objectside, a surface on the object side of the positive lens on the mostobject side being an aspherical surface, and the third group opticalsystem includes one positive lens not including an aspherical surface.10. A photographing lens unit comprising a variable focal length lens asan optical system, the variable focal length lens including a firstgroup optical system having a negative refracting power, a second groupoptical system having a positive refracting power, and a third groupoptical system having a positive refracting power, wherein the firstthrough the third group optical systems are sequentially arranged froman object side; and a stop provided on the object side of the secondgroup optical system and that moves integrally with the second groupoptical system, wherein a focal length is changed by changing relativedistances between the first through the third group optical systems andwhen performing focusing the third group optical system is moved alongan optical axis, the first group optical system includes a negativemeniscus lens, a negative meniscus lens, and a positive lens those aresequentially arranged from the object side, the second group opticalsystem includes a cemented lens of a positive lens and a negative lens,a positive lens, and a positive lens those are sequentially arrangedfrom the object side, the third group optical system includes onepositive lens, at least one surface of the negative meniscus lens in thefirst group optical system and a surface on the most object side in thesecond group optical system being aspherical surfaces, and the thirdgroup optical system includes only a spherical lens.
 11. A cameracomprising a variable focal length lens as a photographing opticalsystem, the variable focal length lens including a first group opticalsystem having a negative refracting power, a second group optical systemhaving a positive refracting power, and a third group optical systemhaving a positive refracting power, wherein the first through the thirdgroup optical systems are sequentially arranged from an object side; anda stop provided on the object side of the second group optical systemand that moves integrally with the second group optical system, whereina focal length is changed by changing distances between the firstthrough the third group optical systems and when performing focusing thethird group optical system is moved along an optical axis, the firstgroup optical system includes a negative meniscus lens, a negativemeniscus lens, and a positive lens those are sequentially arranged fromthe object side, at least one surface of the two negative meniscuslenses being an aspherical surface, the second group optical systemincludes a cemented lens of a positive lens and a negative lens, apositive lens, and a positive lens those are sequentially arranged fromthe object side, a surface on the object side of the positive lens onthe most object side being an aspherical surface, and the third groupoptical system includes one positive lens not including an asphericalsurface.
 12. A camera comprising a variable focal length lens as aphotographing optical system, the variable focal length lens including afirst group optical system having a negative refracting power, a secondgroup optical system having a positive refracting power, and a thirdgroup optical system having a positive refracting power, wherein thefirst through the third group optical systems are sequentially arrangedfrom an object side; and a stop provided on the object side of thesecond group optical system and that moves integrally with the secondgroup optical system, wherein a focal length is changed by changingrelative distances between the first through the third group opticalsystems and when performing focusing the third group optical system ismoved along an optical axis, the first group optical system includes anegative meniscus lens, a negative meniscus lens, and a positive lensthose are sequentially arranged from the object side, the second groupoptical system includes a cemented lens of a positive lens and anegative lens, a positive lens, and a positive lens those aresequentially arranged from the object side, the third group opticalsystem includes one positive lens, at least one surface of the negativemeniscus lens in the first group optical system and a surface on themost object side in the second group optical system being asphericalsurfaces, and the third group optical system includes only a sphericallens.
 13. A portable information terminal device comprising a variablefocal length lens as a photographing optical system of a camera functionunit, the variable focal length lens including a first group opticalsystem having a negative refracting power, a second group optical systemhaving a positive refracting power, and a third group optical systemhaving a positive refracting power, wherein the first through the thirdgroup optical systems are sequentially arranged from an object side; anda stop provided on the object side of the second group optical systemand that moves integrally with the second group optical system, whereina focal length is changed by changing distances between the firstthrough the third group optical systems and when performing focusing thethird group optical system is moved along an optical axis, the firstgroup optical system includes a negative meniscus lens, a negativemeniscus lens, and a positive lens those are sequentially arranged fromthe object side, at least one surface of the two negative meniscuslenses being an aspherical surface, the second group optical systemincludes a cemented lens of a positive lens and a negative lens, apositive lens, and a positive lens those are sequentially arranged fromthe object side, a surface on the object side of the positive lens onthe most object side being an aspherical surface, and the third groupoptical system includes one positive lens not including an asphericalsurface.
 14. A portable information terminal device comprising avariable focal length lens as a photographing optical system of a camerafunction unit, the variable focal length lens including a first groupoptical system having a negative refracting power, a second groupoptical system having a positive refracting power, and a third groupoptical system having a positive refracting power, wherein the firstthrough the third group optical systems are sequentially arranged froman object side; and a stop provided on the object side of the secondgroup optical system and that moves integrally with the second groupoptical system, wherein a focal length is changed by changing relativedistances between the first through the third group optical systems andwhen performing focusing the third group optical system is moved alongan optical axis, the first group optical system includes a negativemeniscus lens, a negative meniscus lens, and a positive lens those aresequentially arranged from the object side, the second group opticalsystem includes a cemented lens of a positive lens and a negative lens,a positive lens, and a positive lens those are sequentially arrangedfrom the object side, the third group optical system includes onepositive lens, at least one surface of the negative meniscus lens in thefirst group optical system and a surface on the most object side in thesecond group optical system being aspherical surfaces, and the thirdgroup optical system includes only a spherical lens.